This invention relates to measuring apparatus for engine ignition systems and more particularly relates to apparatus for measuring the dwell of such an ignition system.
Conventional ignition testers generally employ a single meter movement to measure engine revolutions per minute (RPM), dwell and voltage drop across the ignition points. RPM and voltage drop are generally represented on a scale which has a minimum (e.g., zero) RPM and voltage drop position on the left. As the amount of RPM or voltage drop increases, the pointer of the meter moves to the right.
When such a meter is used to measure dwell, the dwell indication is reversed. That is, the maximum (e.g., 360.degree.) dwell position is on the left and decreasing amounts of dwell cause the pointer to move to the right.
The foregoing operation of conventional dwell-RPM-volt meters creates confusion because the RPM and voltage scales read to the right, whereas the dwell scale reads to the left. Although this problem has plagued the industry for years, no one has been able to devise an accurate and economical meter suitable for use on a variety of ignition systems in which the RPM and dwell scales both read in the same direction.
One attempt to devise such a meter is described in U.S. Pat. No. 3,543,143 (Wanninger -- Nov. 24, 1970). In the Wanninger device a diode is connected in parallel with a meter and a storage capacitor is connected in series with the meter. Although the meter purports to read upscale for increasing dwell, it has a number of disadvantages. Firstly, the forward voltage drop across the diode causes current to flow through the meter while the ignition points are open. This mode of operation is the converse of the desired operation and decreases the accuracy with which relatively high degrees of dwell can be measured. Secondly, the ignition points are in the discharge path of the capacitor. As a result, the accuracy of the dwell indication depends on the voltage across the ignition points during the dwell period. Some ignition systems employing electronic points or switches have a voltage on the order of three volts during the dwell period, whereas ignition systems employing mechanical points have a voltage close to zero during the dwell period. If a Wanninger-type device is used on ignition systems employing electronic and mechanical points, a different dwell indication results even though the actual dwell periods are identical.
Accordingly, it is one object of the present invention to provide a dwell and RPM meter in which both scales read accurately in the same direction when used on different types of ignition systems.
Still another object is to provide a dwell meter in which the pointer is located in a zero position in the absence of current flow through the meter and advances away from the zero position as the dwell of the ignition system increases, and which can accurately indicate dwell on a variety of ignition systems.
Still another object of the invention is to provide a dwell meter of the foregoing type in which the energy produced by an ignition signal during a points open condition is stored and prevented from flowing through the meter by a switch connected in series with the meter. During the points closed or dwell condition, the switch enables at least part of the stored energy to flow through the meter without flowing through the points.
The foregoing objects are achieved by a radical departure from the device shown in the Wanninger patent. Contrary to Wanninger, the present invention does not employ an ammeter connected in parallel with a Zener diode or a diode connected across the ammeter. In general, the principal object of the invention is achieved by providing apparatus including a storage device for storing at least part of the energy in an ignition charging pulse and switching means connected in series with the meter means. The switching means transmits a portion of the stored energy through the meter during the dwell period and inhibits the transmission of energy through the meter while the ignition points are open.